An incident with an infection brought this idea out...
What makes the top species? In our case it's the opposable thumb. It's how we grasp, adding flexibility, agility and strength to our handling of everything from hunting to building shelter, from touch interactions with others -- for good and ill, delicate manipulations, etc.
A species which can bring items close --those two or three feet of keeping the head upright means we're predators too.
You want a species to be the dominant one, they need that capability to bring things close, to throw or toss them away, to hold and all the while have and keep that important view against potential foes.
Having a thumb makes tool use easier, tentacles work much the same way for starfish, octopi and squid in opening recalcitrant shells--so having a tentacle-bearing ruling species isn't out of the realm of possibility.
A common punishment for thieves in the Middle Ages was to deprive the perpetrator of his thumbs. He could still carry heavy loads for honest work, albeit clumsily, but the light-fingered application was severely limited.
The elephant's trunk has 20,000 muscles--tremendous flexibility, sensitivity, reach and weight-bearing capacity. And sitting on one during a summer festival, and she's hot, and she takes a snoot-full of water and snorts it over her back to cool off is an um, not quite an illuminating experience. Of what doused me, I never knew how much was water, how much was elephant snot, and how much was gooey grass and grain from her snack...
The Roman 'thumbs up' live, or 'thumbs down' die gesture is still in use for approval today. 'I bite my thumb' was an insult in Shakespearian times. Hitchhiking needs a thumb to show what way you want to go even if you don't know your destination.
How far can you operate without your thumbs? Tuck them in and type--ok, but that space bar is lonely, and moving the mouse is harder, and grabbing that full coffee cup using the thumb for opposition balance guidance, a little tougher. Eating without a thumb, dressing, driving, lifting, handling the tv remote! All suffer without that odd, sideways mounted digit.
The thumb does rule.